The present invention relates to the signal processing art and, in particular, to a high frequency bandstop/bandpass filter.
Considerable research has been conducted, especially by those in the communication field, into the high frequency filtering of electromagnetic radiation. Conventional designs for high frequency filters include a transmission line to which is coupled one or more open or short circuit stubs. By controlling the electrical length of, and spacing between, stubs the resulting filter provides a desired impedance characteristic as a function of input signal frequency. Both microstrip and strip line fabrication of such microwave stub type filters are well known in the art.
Prior art microwave filter designs which provide both a passband and a stopband characteristic have suffered from a significant defect. Namely, interreactions between various stubs can result in a "flyback" resonance within the stopband, thereby degrading the attenuation characteristics within this band. In some applications the flyback characteristic may be tolerated but, in general, there is a need in the high frequency filtering art for providing a means to suppress the flyback frequency phenomenon.
A need has arisen in the radio frequency mixer art to develop a high frequency bandpass/bandstop filter which does not display a flyback characteristic. There, incoming radio frequency signals are coupled with local oscillator signals in a conventional hybrid coupler. The hybrid coupler output in turn feeds the diodes of a balanced mixer which, in turn, feeds to an intermediate frequency stage. It has been found that the signal to noise ratio of the front end may be significantly enhanced if a filter having a controlled bandstop characteristic at the third harmonic of the local oscillator frequency is placed between the hybrid coupler and each of the mixer diodes. The bandstop characteristic of the filter must, of course, be complemented by a controlled bandpass characteristic at the fundamental of the local oscillator frequency. Due to the aforementioned flyback phenomenon, conventional mixers have suffered a signal to noise degradation since the signals at the flyback frequency have not been filtered prior to application to the mixer diodes.